The development of a water oxidation
catalyst has been a demanding
challenge for the realization of overall water-splitting systems.
Although intensive studies have explored the role of Mn element in
water oxidation catalysis, it has been difficult to understand whether
the catalytic capability originates mainly from either the Mn arrangement
or the Mn valency. In this study, to decouple these two factors and
to investigate the role of Mn valency on catalysis, we selected a
new pyrophosphate-based Mn compound (Li<sub>2</sub>MnP<sub>2</sub>O<sub>7</sub>), which has not been utilized for water oxidation catalysis
to date, as a model system. Due to the monophasic behavior of Li<sub>2</sub>MnP<sub>2</sub>O<sub>7</sub> with delithiation, the Mn valency
of Li<sub>2‑<i>x</i></sub>MnP<sub>2</sub>O<sub>7</sub> (<i>x</i> = 0.3, 0.5, 1) can be controlled with negligible
change in the crystal framework (e.g., volume change ∼1%).
Moreover, inductively coupled plasma mass spectrometry, X-ray photoelectron
spectroscopy, ex-situ X-ray absorption near-edge structure, galvanostatic
charging–discharging, and cyclic voltammetry analysis indicate
that Li<sub>2‑<i>x</i></sub>MnP<sub>2</sub>O<sub>7</sub> (<i>x</i> = 0.3, 0.5, 1) exhibits high catalytic
stability without additional delithiation or phase transformation.
Notably, we observed that, as the averaged oxidation state of Mn in
Li<sub>2‑<i>x</i></sub>MnP<sub>2</sub>O<sub>7</sub> increases from 2 to 3, the catalytic performance is enhanced in
the series Li<sub>2</sub>MnP<sub>2</sub>O<sub>7</sub> < Li<sub>1.7</sub>MnP<sub>2</sub>O<sub>7</sub> < Li<sub>1.5</sub>MnP<sub>2</sub>O<sub>7</sub> < LiMnP<sub>2</sub>O<sub>7</sub>. Moreover,
Li<sub>2</sub>MnP<sub>2</sub>O<sub>7</sub> itself exhibits superior
catalytic performance compared with MnO or MnO<sub>2</sub>. Our study
provides valuable guidelines for developing an efficient Mn-based
catalyst under neutral conditions with controlled Mn valency and atomic
arrangement.